As a conventional spark plug, a spark plug has been known which includes an electrode (a center electrode or a ground electrode) to which an electrode tip (hereinafter referred to as “precious metal tip”) formed of a precious metal or an alloy containing a precious metal as a main component is joined (see Japanese Patent Application Laid-Open (kokai) No. 2013-178912, for example). Generally, a precious metal tip is joined to an electrode base material by laser welding. Specifically, the precious metal tip is irradiated with a laser beam along its outer periphery, whereby the precious metal tip is joined to the electrode base material. When the precious metal tip is welded to the electrode base material, usually, a melt portion in which the material of the precious metal tip and the material of the electrode base material are melted is formed between the precious metal tip and the electrode base material.
As described above, in the spark plug including the precious metal tip, at an interface (hereinafter also referred to as a melt portion interface) between the melt portion and the precious metal tip, an oxide film (hereinafter also referred to as an oxide scale) may be formed on the surface of the melt portion. The oxide scale is formed, at the melt portion interface, so as to gradually grow from an outer peripheral portion near the outside air toward the inside of the melt portion interface.
When the spark plug is used, heating and cooling cycles are repeated, whereby a stress is caused by a difference in thermal expansion coefficient between the precious metal tip and the electrode base material, near a joint portion of the precious metal tip and the electrode base material. Generally, an oxide scale is lower in strength (is more fragile) than the melt portion or the precious metal tip. Therefore, when a stress occurs as described above, crack is likely to occur in the oxide scale having the relatively low strength. When crack occurs in the oxide scale and the air enters the crack, oxidation of the melt portion interface progresses, and the oxide scale further grows toward the inside of the melt portion interface. The growth of the oxide scale toward the inside of the melt portion interface causes the crack to extend toward the inside of the melt portion interface, leading to falling off of the precious metal tip, which makes it difficult to secure reliability of the joint between the precious metal tip and the electrode base material.
Conventionally proposed measures to improve the reliability of the joint between the precious metal tip and the electrode base material are: forming the melt portion to be thicker; and adjusting the shape of the melt portion to suppress the stress that occurs between the precious metal tip and the electrode base material (refer to Patent Document 1, for example). Since the melt portion has an intermediate composition between the precious metal tip and the electrode base material, a difference in thermal expansion coefficient between the precious metal tip and the melt portion is smaller than the difference in thermal expansion coefficient between the precious metal tip and the electrode base material. Therefore, for example, by increasing the thickness of the melt portion, a stress that occurs near the interface between the precious metal tip and the melt portion can be suppressed, and crack is suppressed from occurring in the oxide scale due to the stress.
However, measures to suppress growth of the oxide scale at the interface between the precious metal tip and the electrode base material have not been sufficiently investigated. Therefore, it has been desired to suppress growth of the oxide scale and improve the reliability of the joint between the precious metal tip and the electrode base material.